Level: * Difficult      ** Very difficult      *** Extremely difficult

Hint: # Trivial hint. Check it after 10 minutes.      ## Moderate hint      ### Substantial hint. But still require a lot of thinking.

I will check the correctness of your solution if you email it to .
I will make it publicly available and acknowledged under your name upon your consent.

Note: all problems are well-defined and rigorously solved. No wording or semantic tricks exploiting metaphor or ambiguity.

* Click here for Difficult Problems

** Click here for Very Difficult Problems

*** Click here for Extremely Difficult Problems

Some problems are used during job interviews by technology and financial companies.

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